The present invention relates to a method for modifying surface properties of a shaped article of a vinyl chloride-based resin or, more particularly, to a method for reducing bleeding or blooming of a plasticizer or other additive ingredients contained in a shaped article of a vinyl chloride-based resin on the surface of the article by the treatment with low temperature plasma.
Needless to say, vinyl chloride-based resins belong to one of the most important classes of thermoplastic resins owing to their excellent and very versatile properties along with their relative inexpensiveness so that they are widely employed in a variety of application fields as shaped in various articles. In particular, rigidity or flexibility of shaped articles of vinyl chloride-based resins can be adequately controlled by formulating the resin with a plasticizer. The thus plasticized resins can give relatively flexible shaped articles so that plasticized resin compositions are fabricated into films, sheets, synthetic leathers, tubes, hoses, bags, coating materials and the like used in various fields such as medical instruments, packaging materials for foodstuffs, materials for agricultural use, building materials and the like.
Furthermore, it is a very common practice that vinyl chloride-based resins are formulated with various kinds of additive ingredients such as flame retardants, anti-oxidants, ultraviolet absorbers, lubricants and others according to the particular needs for the improvements of the workability of the resin composition as well as the properties of the articles shaped with the resin composition.
One of the most serious problems in these articles shaped with the vinyl chloride-based resin composition formulated with the additive ingredients, typically, a plasticizer, is that the plasticizer contained in the shaped article may sometimes migrate toward the surface of the article and exude on the surface of the article in the long run resulting in inferior properties of the plasticized vinyl chloride-based resin articles. This phenomenon is usually called "bleeding," prevention of which is one of the most important problems difficult to solve in the technology of synthetic resin processing.
Bleeding of the plasticizer is undesirable not only due to the deterioration of the properties of the shaped articles but also due to the transfer of the plasticizer exuded on the surface of an article to the surface of the other body being in contact with the shaped article of the plasticized vinyl chloride-based resin composition. Along with the loss in the beauty of the surface appearance of the articles, bleeding or transfer of the plasticizer is especially undesirable when the shaped article is used in a medical instrument or in contact with foods because safety of plasticizers to the human health is not established in general so that the use of plasticized vinyl chloride-based resin compositions in these fields is largely limited. Similar problems are involved in the use of shaped articles containing the other kinds of additive ingredients.
Various attempts have been made to solve the above described problem of surface bleeding of plasticizers and the like. The methods hitherto proposed for the purpose include a method of treatment or irradiation of the surface of the articles with ionizing radiation, high-energy electron beams or ultraviolet light, a method of providing coating films on the surface of the articles with a suitable synthetic resin capable of preventing surface bleeding of the plasticizer and a method of chemical treatment in which certain special chemicals are applied on to the surface of the articles or admixed in the composition of the vinyl chloride-based resin for shaping the articles. These methods are, however, not satisfactory in most cases in the effectiveness with, instead, an adverse effect of impairing several advantageous properties inherently possessed by the vinyl chloride-based resins.
For example, irradiation with ionizing radiation or high-energy electron beams may produce crosslinking between the polymer molecules not only in the superficial layer of the shaped article but also in the subsurface layer due to the excessively high energy of the radiation so that the flexibility as the most characteristic feature of the plasticized vinyl chloride resin articles is largely lost. Treatment with ultraviolet light is undesirable by the reason of coloring in the surface due to the degradation of the polymer molecules in the surface layer of the shaped article. The chemical means is not free from the problems of surface erosion of the shaped articles and the poor adhesivity and durability of the films provided on the surface of the article.
Recently, it has been proposed that the surface bleeding of a plasticizer in a shaped article of a vinyl chloride-based resin can be prevented by exposing the article to an atmosphere of low temperature plasma of a gas such as rare gases, oxygen, nitrogen, carbon monoxide and the like of low pressure generated by glow discharge. This method of plasma treatment is effective for preventing migration and surface bleeding of the plasticizer by the formation of highly crosslinked layer only in the very surface of the article without affecting the desirable properties of the article as a bulk. It has been shown by the inventors that the plasma treatment of shaped articles of vinyl chloride-based resins is also effective in improving the wettability, adhesivity and printability as well as in reducing electrostatic charging and surface stain.
When the object of the plasma treatment is the improvement of the properties other than plasticizer bleeding as mentioned above, no rigorous control of the conditions is required in the plasma treatment. On the other hand, the prevention of plasticizer bleeding on the surface by the plasma treatment can be achieved only by a very careful control of the treatment conditions for the individual articles. For example, when the plasma treatment of a plasticized polyvinyl chloride shaped articles is undertaken with varied length of treatment time, the amount of the plasticizer bleeding first decreases with the increase of the treatment time but, after reaching a minimum point, the amount of the plasticizer bleeding again increases with the increase of the treatment time. This phenomenon presents a difficult technical problems of determining the optimum treatment time for the individual articles. Further, the effect of the plasma treatment has a threshold value below which no expected effect can be obtained with the plasma treatment and the effect of the plasma treatment appears only when the treatment time exceeds the threshold.
The above mentioned optimum range of the treatment time becomes narrower and narrower as the electric power of the plasma discharge is increased with an object of decreasing the overall treatment time presenting an unsurmountable difficulty. When the electric power for the plasma discharge is extremely large, practically no effect of prevention of the plasticizer bleeding can be obtained. These circumstances of poor reproducibility greatly lessen the practical value of the plasma treatment for preventing the plasticizer bleeding.
The above described disadvantages of the plasma treatment method take place irrespectively of the parameters of plasma conditions such as kind, pressure and flow rate of the plasma gas and the formulation of the vinyl chloride-based resin compositions resulting in reduced practical values of the method, especially, in the rapid or continuous treatment of general purpose shaped articles of the resin compositions with low temperature plasma.